1. Field of the Invention
The present invention relates generally to electron paramagnetic resonance (EPR) resonance spectroscopy systems and, more particularly, relates to improved resonators for use in such systems.
Resonator coils are used to detect magnetic resonance responses from infusible or implantable spin probes in living objects after pulsed excitation using radiofrequency, in the range 60-400 MHz, of the sample placed in a magnetic field. With the addition of gradient fields to the stationary magnetic field, the responses are collected for image reconstruction purposes.
2. Description of the Relevant Art
In electron paramagnetic resonance (EPR) spectroscopy, a uniform magnetic field Bo is applied to an object to align the magnetic moments of the electrons along the Z-axis of the applied magnetic field (C. P. Poole Jr, Electron Spin Resonance, 2nd Ed., Wiley, NY). An EPR spectroscopy system is described in a co-assigned patent application to Murugesan et al. (Ser. No. 08/504,616 filed Jul. 20, 1995) which is hereby incorporated by reference for all purposes. For a single electron system the spins are in either of the two levels, a higher and lower energy level. Electromagnetic radiation of appropriate frequency can cause absorption of energy by inducing transitions between the two states, a process called electron paramagnetic resonance (EPR). This process is similar to nuclear magnetic resonance where the nuclear spin systems such as protons are studied in a similar manner.
The frequency of operation is related to the magnetic moment of the spin system, and the applied magnetic field or the frequency of the incident RF radiation and is given by the equation:
2xcfx80xcexd0=xcexdeB0
where xcexd0 is the frequency of operation, B0 is the applied magnetic field, and xcexde is the gyromagnetic ratio of the spin system. Since the g for electrons is nearly 660 times greater in magnitude than that of a proton, a correspondingly lower applied magnetic field compared to proton NMR is necessary for a given frequency of operation.
After radiofrequency irradiation pulses of suitable duration and intensity are used to irradiate a sample containing unpaired electrons in a resonator coil, weak resonance signals, called xe2x80x9cfree induction decays,xe2x80x9d (FIDs) which decay in amplitude as a function of time, are detected by the same resonator coil, which also serves as a receiver. The FID is converted to a resonance absorption signal by mathematical transformation called the Fourier transformation.
For proton NMR, the pulse widths of irradiation are in the microsecond range and the FIDs last for times ranging from milliseconds to seconds. For free electron spin probes such as free radicals, the corresponding excitation pulse widths are typically in the range of 10 to 100 ns, and the FIDs last between 100 ns to 20 xcexcs.
The purpose of the resonator coil is to deliver the transmitted power into the spin system to create and receive resonance signals.
Coupling the input power efficiently into the spin system and recovering the resonance signals at optimal strength will have a significant influence in the quality of images obtained. Several resonators exist for Electron Paramagnetic Resonance (EPR) practiced at frequencies in the range of 9 GHz and above (Pfenninger et. al., xe2x80x9cGeneral Method for Adjusting the Q-factor of EPR Resonators,xe2x80x9d Rev. Sci. Instr. vol 68, 4857-4865, 1995; Prisner et. al., xe2x80x9cPulsed 95 GHz High Field EPR Heterodyne Spectrometer with High Spectral and Time Resolution,xe2x80x9d Appl. Magn. Reson. 7, 167-183 1994). These resonators are not suitable for studies involving living objects due to the small volumes available in which to place the object under study. The resonators used in MRI using protons as spin probes are suitable in terms of size but not in terms of electrical characteristics such as the resonator dead time (R S Withers and G C Liang, U.S. Pat. No. 5,276,398 dated Jan. 4, 1994).
The dead time is the time taken for the noise originating from the intense input power to the resonator housing the object under study to return to thermal noise levels and is given by the equation:
TD=t Q/2xcfx80xcexd0
where t is the time constant for the noise dissipation, Q is the quality factor of the resonator and xcexd0 is the operating frequency (Pfenninger et. al., xe2x80x9cGeneral Method for Adjusting the Q-factor of EPR Resonators,xe2x80x9d Rev. Sci. Instr. vol 68, 4857-4865, 1995). Only after the resonator dissipates the noise associated with the intense input power can the rapidly decaying weak signals associated with the resonance absorption be recovered.
The dead times are not an important factor in MRI, since the signals associated with the resonance absorption of the nuclei are long lived (in the order of seconds) permitting dead times in the order of micro- to milliseconds. In addition, in MRI, since the spectral bandwidth is extremely narrow ( less than 10 kHz), resonators of high Q-values are desirable for enhanced sensitivity (R S Withers and G C Liang, U.S. Pat. No. 5,276,398 dated Jan. 4, 1994).
For time domain EPR experiments, the lower the frequency, the longer is the time required for the receiver to recover after the dead time. In addition, the spectral band width of EPR is in the order of 5-10 MHz. These two factors limit the resonator Q-values to less than 50.
The input power (P) to be given to the spin system to provide a given magnitude of the magnetic field B1 is dependent on both the Q-value of the resonator and the volume V of the resonator and is given by the expression:
P=constant (V/Q).
Accordingly, spin systems in resonators with lower Q values provide correspondingly lower resonance signals compared to resonators having higher Q values. In addition, the increased volumes necessary for studying living objects necessitates the use of resonators with correspondingly increased volumes. This causes an added demand on the input power to achieve a given magnetic component (B1) of the RF (Prisner et. al., xe2x80x9cPulsed 95 GHz High Field EPR Heterodyne Spectrometer with High Spectral and Time Resolution,xe2x80x9d Appl. Magn. Reson. 7, 167-183 1994).
Lowering the Q-value by overcoupling has been suggested to be desirable over resistively lowering the Q for resonators for EPR experiments conducted at X-band frequency so as to minimize the Q-value without decreasing the effective B1 field at a given input power (Rinard et. al., xe2x80x9cRelative Benefits of Overcoupled Resonators vs. Inherently Low-Q Resonators for Pulsed Magnetic Resonance,xe2x80x9d J. Magn. Reson. A 108, 71-81, 1994). Two helical coils connected in parallel and overcoupled to lower the Q have been implemented for NMR (Chingas, xe2x80x9cOvercoupling NMR Probes to Improve Transient Response,xe2x80x9d J. Magn. Reson. 54, 153-157, 1983). However, since the demands of time domain RF EPR in terms of decreased dead times, effective B1 field, RF penetration and enhanced sensitivity, the available designs were found not to be adequate.
An object of the present invention is to provide a family of resonator coils necessary to accommodate and irradiate objects of interest of varying dimensions, such as living objects, containing free radical spin probes and induce an EPR signal which can also be recovered by the resonator. Such a resonator has the capability of facilitating the enhanced dissipation of noise to thermal noise levels associated with the input power from the RF pulse, and recovering weak and rapidly decaying FIDs. In addition, the lowering of the Q values by overcoupling, instead of resistively damping provides enhanced B1 fields thereby increasing the sensitivity of detection of the resonance signals after pulsed excitation.
According to one aspect of the invention, a radiofrequency (RF) coil design suitable for detecting time domain electron paramagnetic resonance (EPR) responses from spin probes after pulsed excitation using radiofrequency irradiation (60-400 MHz) is provided. The coil is configured in an array of numerous surface coils of appropriate diameters connected in a parallel configuration with appropriate spacing between individual surface coils to form a volume type resonator. Such resonators are suitable for detecting time domain EPR signals after pulsed excitation from spin probes in a living object. Such resonators can also be utilized for collecting EPR responses under gradient magnetic fields and processed for image reconstruction. Since the dead time of the resonator and the spectral window provided by the resonator are critical for the success of the imaging techniques using time domain EPR experiments, the resonator design provides for a lower dead time and broad band Q profile as well as acceptable uniformity in RF irradiation to provide images of free radical distribution from a living object or objects within the resonator (paper entitled xe2x80x9cIn vivo Imaging of a Stable Paramagnetic Probe by Pulsed-Radiofrequency Electron Paramagnetic Resonance Spectroscopy,xe2x80x9d Murugesan et. al., Magnetic Resonance in Medicine, 1997).
According to another aspect of the invention, coils are oriented axially with respect to a cylindrical tube and coupled in parallel to receive RF energy.
The present invention has abilities to effectively couple the input power to the spin system, shorten the dead time and also pick up the weak and rapidly decaying resonance signals associated with free radical spin probes. The design provides ability to fabricate resonators of appropriate dimensions to study living objects.
According to another aspect of the invention, Q-switching is utilized to increase the sensitivity of the resonator.
Other features and advantages will be apparent in view of the following detailed description and appended drawings.